"The economic picture looks pretty similar to the last couple of months, which helps explain why the industry has stayed in a relatively healthy range," Kurt McNeil, head of GM's US sales operations, said during a conference call.

"Business spending has picked up and pent-up demand for vehicles is offsetting any drag from tax or federal spending issues," he added.

Ford beats, GM misses

Auto sales each month are an early indicator of economic health. The auto industry is in the middle of its fourth year of recovery from an economic downturn that pushed GM and Chrysler into bankruptcy in 2009.

Analysts polled by Thomson Reuters were expecting an annual sales rate of 15.3 million vehicles in March. GM estimated the rate would come in around 15.2 million.

GM's results fell short of the estimates of at least three analysts. GM sold 245,950 cars and trucks in March, up 6.4 percent from a year earlier.

TrueCar.com analyst Jesse Toprak said GM's miss came from lower-than-expected car sales and fleet business.

Ford sales rose 5.7 percent to a stronger-than-anticipated 236,160 vehicles, making March its best month since May 2007. Toyota Motor Corp posted a 1 percent US sales increase to 205,342 vehicles.

For Chrysler Group LLC, the No 3 US automaker, it was the best month since December 2007. Sales at Chrysler, a unit of Italian automaker Fiat SpA, rose 5 percent to 171,606 cars and trucks.

The US auto market is among the strongest in the world and is increasingly critical for major automakers as car sales in European tumble. Sales in Spain, for example, fell 13.9 percent in March, figures released on Tuesday showed.

Several analysts have raised their full-year US sales forecasts in the last week. GM stuck to its forecast for industry sales as high as 15.5 million this year, which would be the best year since 2007, before the US economy slid into recession.

The industry's swift recovery over the last few years has allowed the sector to be a bright spot during an uncertain economic recovery. But Toprak said those outsized gains would be difficult to maintain.

"After this year, it's going to become a bit tricky," he said. "Getting to 15.5 (million) is not going to be as tough as getting to 16.5 (million). Getting there may take another couple of years."